1. Field of the Invention
The present invention relates generally to methods and apparatus for measurement of very small circuit resistances (0.025 ohm or less) without disconnection of the circuit and particularly to an improved version of the apparatus previously developed by the inventor. The earlier version of the method and apparatus are described in U.S. Pat. No. 5,189,375. This improved version, though similar in some respects to the earlier patented version, employs a different measurement principle which extends the range of resistances that can be measured. In addition, individual resistances within the circuit under test can now be measured, which is an improvement over the earlier version which measured total circuit resistance. Further, the inductance of the circuit can also be measured without disconnection.
2. Description of the Prior Art
Electromagnetic Pulse (EMP) hardness assurance maintenance and surveillance programs and system life cycle study programs have been established by all branches of the military which have fielded systems with EMP survival requirements. A major portion of these programs is concerned with measurement of cable shield degradation over the lifetime of the system so that short and long term maintenance actions can be planned. The systems which require these support programs include mobile and fixed C3 (Command, Control and Communications) systems, combat vehicles (battle tanks), aircraft, and naval vessels. To support these programs, computer based data collection systems featuring network analyzers have been built or are planned. These Computer controlled network analyzers provide the capability to collect, process, and store large amounts of high quality data in the form of transfer impedance (Zt) vs frequency plots from 5 Hz to 200 MHZ. Such systems have been configured into depot testers used primarily by the Air Force logistics command. Although the use of Zt vs frequency plots in a 200 MHz bandwidth allows the data analyst to diagnose cable connector problems and to track gradual deterioration of cable shields over the lifetime of the cable, the method has several drawbacks:
First, the cable must be disconnected at one or both ends so that access to interior conductors can be gained. Disconnection of a cable from its equipment (though temporary) may eliminate a flaw or a source of degradation from the measurement environment, consequently, a flaw may go undetected.
Secondly, Zt vs frequency plots are time consuming; this is especially significant when the system to be tested is a major aircraft such as a bomber (B52) or intelligence, reconnaissance, surveillance aircraft (AWACS). Such aircraft reportedly have up to 600 cables some of which are multiple branch cables. Further, such major systems are available for maintenance testing infrequently and for very short time intervals. This translates to incomplete testing because of insufficient time or unacceptable delays in scheduling other tests on the aircraft by other organizations.
Thirdly, network analyzer based depot testers are operated by highly skilled contractor personnel who must travel to the various sites and depots where testing must be performed.
Fourthly, the initial cost of such systems is high.
The technique of inductively coupled low resistance measurement is proposed as a very inexpensive complement to the more orthodox methods. Since the subject invention can be inexpensively mass produced and can be easily used by relatively unskilled personnel, it may be cost effectively distributed to local maintenance organizations. Many of the problems detectable via the subject invention can be repaired on the spot by such actions as cleaning threads on a connector backshell or tightening the coupling nut of a panel connector. With additional effort, the method and apparatus can be adapted to the inspection of conductors in the grounding systems of computer and communications facilities (fixed sites) and to the inspection of metal conduits.
Flaws in the shields of cables can usually be traced to connectors. These flaws which can result from improper bond between cable shield and connector backshell, mechanical stress, or metal oxide buildup at connector junctions, introduce resistances in series with the cable shield and reduce the overall effectiveness of the shield. When such flaws are present, they can be sometimes detected by the above described measurements of transfer impedance vs frequency obtained with a network analyzer or by direct measurement of cable shield resistance obtained with a milliohmeter. The presence of a flaw in the shield will be indicated by an increase in transfer impedance (ohms/length) or shield resistance (ohms) above a previously established maximum allowable value. Typical acceptable values of shield resistance of cables in real systems will range from ten milliohms (0.01 ohm) to several tenths of ohms depending upon such cable parameters as length, diameter, characteristics of the shield material, and allowable junction resistances.
It is not always desirable to attempt detection of cable shield flaws by measurements of transfer impedance or shield resistance by the standard techniques. Both techniques require that the equipment terminating the cable under test be disconnected. When measurements are made on a cable disconnected from its equipment, a serious flaw may go undetected. Disconnection may relieve the mechanical stress that caused the flaw or may eliminate a resistive junction between cable connector and equipment connector. Also, the shield flaw may exist, not within the cable, but at the junction between equipment connector and equipment enclosure. Thus, inspection for flaws in cable shields should be done with the cable connected to its terminating equipment so that all sources of shield degradation will be present in the measurement environment. It should also be stated that flaws can develop in cables which may decrease the shield resistance. Such flaws occur when a cable shield becomes shorted to the wall or floor of the enclosure (vehicle or aircraft) in which the cable is located. This type of flaw has been known to occur in aircraft when the cable shield insulation is severed due to mechanical stress. This type of fault effectively changes the length of a cable and this condition can be identified by a decrease in circuit inductance. Therefore, detection of flaws and degradation in cable shields translates to location and measurement of low valued resistances and inductances without disconnecting the circuit under test.
Accordingly, it is an object of the present invention to provide an inductively coupled low resistance measurement method and apparatus which can be used without disconnecting the circuit under test.
It is another object of the present invention to provide an inexpensive cable test set that works in combination with a user supplied portable oscilloscope that provides a continuous display of a pulsed current waveform induced on a cable by the test set and/or a continuous display of a pulsed voltage waveform that is developed across resistive junctions due to the induced current.